1. Technical Field
The present invention relates to a fluorinated elastic copolymer and a method for its production. Particularly, it relates to a fluorinated elastic copolymer which is excellent in crosslinking reactivity and has high fluidity, and which is crosslinkable to obtain a crosslinked rubber excellent in permanent compression set and base resistance, and a method for its production.
2. Background Art
Fluorinated elastic copolymers are excellent in heat resistance, chemical resistance, oil resistance, weather resistance, etc. and thus can be applied to a severe environment wherein usual hydrocarbon type materials are not durable. As such fluorinated elastic copolymers, a vinylidene fluoride/hexafluoropropylene copolymer, a tetrafluoroethylene/propylene copolymer, a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer, etc. are known.
Among them, a tetrafluoroethylene/propylene copolymer is excellent in amine resistance and high temperature vapor resistance and also excellent in an electric insulation property, as compared with the above fluorinated elastic copolymer containing repeating units derived from vinylidene fluoride, and thus has been widely used as e.g. a covering material for electric wires (Patent Documents 1 and 2). However, such a tetrafluoroethylene/propylene copolymer has such a nature that it is likely to be swollen by an apolar solvent such as gasoline or automatic transmission oil and thus has had a problem that its use is limited under such an environment where it is exposed to such a solvent atmosphere.
These fluorinated elastic copolymers are poor in reactivity, and their crosslinking reactivity is not sufficient, and heretofore, methods of introducing reactive functional groups to improve the reactivity have been proposed. Among them, in order to improve the crosslinking reactivity by a peroxide, a method of copolymerizing a special curable monomer, a method of carrying out polymerization in the presence of a chain transfer agent having a reactive functional group, a method of pretreating a fluorinated elastic copolymer before crosslinking by a peroxide, etc. have been practically adopted. For example, a fluorinated elastic copolymer obtained by copolymerizing tetrafluoroethylene and propylene in the presence of a fluorinated chain transfer agent having an iodine atom, has iodine atoms at the polymer terminals, whereby crosslinking with a peroxide is possible (Patent Document 3).